how do i add gloss to, let's say, just over the nose?
I'm only fairly new to Blender, but I believe that if you paint the area you want glossy in Texture Paint mode with a new black image as the base (and the glossy part painted white) you can then save that image. Then create a Gloss Node with your newly created image as the Fac input and use a Mix Node to mix it with your skin node/s.